1. Technical Field
The invention covers simultaneous bidirectional (duplex) data transmission field, and especially QAM or phase modulation, over a same media. The specific subject is an echo suppressor for such a system, of the type including an adaptive digital filter designed to provide an estimate of .sigma..sub.k (the effective echo) and whose adaptation algorithm of vector C of coefficients has the form: EQU C.sub.k+1 =C.sub.k +.mu.a.sub.k *.f(e.sub.k)
wherein .mu. and f are a determined incrementation step and a predetermined function;
2. Description of the Prior Art
Before presenting the state of the prior art and the contribution of the invention, it may be useful to recall some information related to data bidirectional transmission over a same medium and the relevant problems.
FIG. 1 shows the schematic diagram of a simultaneous bidirectional transmission system between two remote terminals A and B, over a same transmission medium 10 that may be for example a two-wire telephone line. The information to be transmitted consists of a sequence of symbols, generally quantified, that may represent data signals as well as speech signals, when useful signals a and d coming from terminals A and B are transmitted within the same bandwidth, signal y received in receiver 12 of terminal A includes the useful signal d (remote data) generated by transmitter 11 of remote terminal B, lost however in additive noise: EQU y:d+n+.sigma.' (1)
The noise is often of a higher level than useful signal d, it includes additive line noise n and the echo .sigma.' of signal a (local data) sent by terminal A, and this in spite of the presence of differential transformers 15 at both ends of the transmission media 10. This phenomenon is schematically illustrated in FIG. 1 where transmission from B to A is shown in solid lines, whereas transmission from A to B is shown in dashed lines. The preponderant part of this noise is generally echo .sigma.', which derives from local data through the unknown "echo filter" C.sub.o : EQU .sigma.'.sub.k =C.sub.o.a.sub.k.
It is necessary to cancel, or at least attenuate the action of the echo which would prevent the recovery of signal d in receiver 12 of terminal A. Many echo suppression techniques have been proposed. The most common solution has been to insert at each station an adaptive digital filter 13, so-called "echo canceller", with a transfer function represented by a vector of coefficients C.sub.k, which from a sequence of successive symbols a.sub.k (k indicating the symbol sequential number) transmitted by source 11 in station A, sequence obviously available in station A, provides a linear estimation EQU .sigma..sub.k =C.sub.k .multidot.a.sub.k ( 2).
The estimation is called recovered echo or estimation of true echo .sigma.'. This recovered echo is sent to substractor 14 that receives also the signal y sent to station A through line 10. The difference e.sub.k between the two signals EQU e.sub.k =y.sub.k -.sigma..sub.k ( 3)
is applied to receiver 12.
The power of echo .sigma..sub.k ', is much variable, as well as the power of useful signal d.sub.k.
The data used is, as a rule, complex (case of phase modulation and modulation with two carriers in quadrature. In this case the adaptation algorithm used typically has the following form: EQU C.sub.k+1 =C.sub.k +.mu..a.sub.k *.f(e.sub.k) (4)
wherein .mu. and f are respectively an incrementation step and a predetermined function.
This formula assumes, as a representation of residual echo er.sub.k, the "clean" signal EQU e.sub.k =tr.sub.k +d.sub.k +n.sub.k
wherein er.sub.k =(C.sub.o -C.sub.k).a.sub.k.
Most of the time, when the echo does not exhibit a significant phase shift, the function f adopted is the gradient of the quadratic error, so that the algorithm (4) becomes: EQU C.sub.k+1 =C.sub.k +.mu.(y.sub.k -.sigma..sub.k)a.sub.k * (5).
The shortcoming of the above solution is to require a complex filter, with a great number of bits for each coefficient--namely about twenty--and heavy computation.
Therefore it has been envisioned to use the sign of e.sub.k =y.sub.k -.sigma..sub.k as a function, instead of the gradient itself, in order to simplify the implantation. But it is found that this algorithm cannot ensure a full convergence of the adaptation.
Actually this algorithm is written as: EQU C.sub.k+1 =C.sub.k +.mu..a.sub.k *.sign e.sub.k ( 6)
wherein EQU sign.(e.sub.k)=1/.sqroot.2[signe.sub.k.sup.r +i signe.sub.k.sup.i ](7)
e.sub.k.sup.r and e.sub.k.sup.i being respectively the real and imaginary components of e.sub.k.
To obtain convergence, e.sub.k must have the same sign as the residual echo er.sub.k. However, as soon as the adaptation is sufficient to have the level of er.sub.k lower than the remote data level d.sub.k, the sign identity condition is not satisfied anymore and the echo residue convergence is interrupted (T.A.C.M; CLAASEN, W. F. G. MECKLENBRAUKER, "Comparison of the convergence of two algorithms for adaptive FIR digital filters", TEEE Trans. ASSP, Vol. 29, No. 3, June 1981, pp. 670-678).
This difficulty is shown in FIG. 2 where is illustrated the case with opposite signs of the real components of residual echo er.sub.k and of the difference e.sub.k between the recovered echo .sigma..sub.k and the received signal d.sub.k.
Other solutions have been used to solve this problem. For example it has been proposed to insert a forced noise signal b.sub.k, with a kown level close to the level of remote data d.sub.k, in order to compensate for the presence of the data (FIG. 1). However, this adjunction significantly increases the system complexity (N. HOLTE, S. STUEFLOTTEN, "A new digital echo canceler for two-wire subscriber lines"; IEEE Trans. on Communications, Vol. COM-29, No. 11, Nov. 1981, pp. 1573-1581).